Mesoporous oxides having a variety of compositions and uniform pore sizes have been made with the use of organic templating or directing agents.
Silica-based mesoporous materials, e.g., M41S, have been prepared by organizing silica with organic surfactants (See C. T. Kresge et al., Nature 1992, 359, 710-712 and J. S. Beck et al., J. Am. Chem. Soc. 1992, 114, 10834-10843). These materials can exhibit cubic or hexagonal symmetry, e.g., MCM-48 and MCM-41, respectively. Thermal decomposition of the surfactant allows for the development of narrow pore size distributions in the range 15-100 Angstroms and BET specific surface areas above 1000 m2/g.
Mesoporous materials are not restricted to silica. For example, MCM-41 type materials have been reported for oxides of titanium (See D. M. Antonelli et al., Angew. Chem. Int. Ed. Eng. 1995, 34, No. 18, 2014-2017), antimony, and lead (See Q. Huo et al., Nature 1994, 368, 317-321 and Q. Huo et al., Science 1995, 269, 1242-1244). Mesoporous crystalline materials containing such oxide materials as silica and alumina have been described (See, for example, U.S. Pat. No. 5,057,296 to J. S. Beck and U.S. Pat. No. 5,198,203 to C. T. Kresge et al.).
Mesoporous, alumina compositions having an average pore diameter substantially ranging from about 15 Angstroms to about 40 Angstroms and an average surface area of no less than about 500 m2/g are described in the Davis et al U.S. Pat. No. 5,863,515. These materials can be formed by treating an aluminum source that is derived from an aluminum alkoxide in an organic-aqueous solution with an organic structured directing agent to form meso-sized micelles followed by recovery and calcination of the resulting solid composition. Organic structural directing agents include anionic surfactants, such as alkyl carboxylic acids.
Semi-crystalline alumina compositions with framework mesopores, characterized as the MSU-X series of materials, are described in the Pinnavia et al U.S. Pat. No. 6,027,706 and the Zhang et al article in Chem. Commun, 1998 1185. These materials are said to be semi-crystalline and exhibit an X-ray diffraction peak at less than 5 degrees 2×theta (CuKα). The compositions are made from inorganic aluminum precursors and nonionic surfactants, such as polyethylene oxide compositions. Recovery of the surfactant can be achieved through solvent extraction where the solvent may be water or ethanol. The surfactant occluded in the pores of the as-synthesized materials may also be removed by calcination.
The Liu et al U.S. Pat. No. 6,797,248 describes a mesoporous molecular sieve, MPL-1, and its preparation process. The anhydrous composition of this molecular sieve contains at least three elements, i.e. aluminum, phosphorus and oxygen. The molecular sieve has large pore diameters, generally in the 1.3 nm-10.0 nm, a large specific surface area and adsorption capacity. MPL-1 is synthesized under a hydrothermal process with an organic compound as template. Where necessary, silicon and/or titanium may be added to synthesize an aluminosilicophosphate, aluminotitanophosphate, or aluminosilicotitanophosphate molecular sieves having a mesoporous structure. Other metal compounds may also be added to synthesize derivatives of mesoporous aluminophosphate molecular sieves containing the corresponding hetero-atoms. The other metal element in addition to aluminum, which may be used in the molecular sieve may be one or more selected from the group consisting of La, Ce, Ti, Ni, Co, Cr, Ca, Cu, Zn, Mg, and Fe. The template used may be represented by the general formula: R1R2R3R4NX, wherein R1, R2, R3, and R4 independently represent a substituting group, N represents the element nitrogen or phosphorus, and X represents hydroxyl or halogen such as F, Cl, Br, or 1. At least one substituting group among R1, R2, R3, and R4 is a group containing 5 or more carbon atoms, such that R1R2R3R4NX represents cetyl trimethylammonium chloride (CTMAC), cetyl trimethylammonium bromide (CTMAB), octadecyl trimethylammonium salts. At least one substituting group among R1, R2, R3, and R4 may contain one or more polar functional groups, which can be selected from a group consisting of amino, hydroxyl, carboxyl, sulfhydryl, aldehyde group, and halogens such as F, Cl, Br or 1. Examples of templating agents include phenethoxy-2-hydroxypropyl trimethylammonium chloride (PTMAC) and/or phenethoxy-2-hydroxypropyl trimethylammonium bromide (PTMAB).
The Kolenda et al U.S. Pat. No. 6,214,312 describes the preparation of oxides with a controlled porosity in which an alumina precursor is prepared by hydrolysis of at least one anionic inorganic source of aluminium in the presence of at least one surfactant. When the structuring agent is a quaternary ammonium type, the X ray diffraction pattern generally exhibits a single diffuse diffraction below 5 degrees 2×theta (CuKα). The structuring agent may be a cationic surfactant or a mixture of a cationic surfactant and an anionic surfactant, wherein the mixture has a net positive charge.
The Lee et al U.S. Patent Application Publication No. US 2005/0281734 A1 describes a mesoporous alumina molecular sieve made by mixing a surfactant and an alumina precursor with an organic solvent to produce a mixture, adding water to the mixture, hydrothermally synthesizing the mixture with added water, and then drying and calcinating the mixture to remove residual surfactants. Any of the surfactants, such as a quaternerary ammonium surfactant, commonly used in the field may be used. The alumina precursor may be an aluminum alkoxide, such as aluminum tri-sec-butoxide or aluminum isopropoxide.
The Shan et al U.S. Pat. No. 7,211,238 and the Shan et al article in Applied Catalysis A: General 254 (2003), pp. 339-343 describe mesoporous aluminum oxides with high surface areas, characterized as Al-TUD-1, which are synthesized using small organic templating agents instead of surfactants. Examples of such templating agents include tetraethylene glycol, triethanolamine, triisopropanolamine, triethylene glycol, diethylene glycol, sulfolane, and diethylglycoldibenzonate. The aluminum source for preparing the mesporous oxide may be mixed with a framework substituted element selected from the group consisting of Si, Ga, B, P, S, La, Ce, Ti, Fe, Ni, Mo, Co, Cr, Mg, Zn, Sn, V, W, Ru, Pt, Pd, In, Mn and Cu.
In the above-mentioned publications, regarding the manufacture of mesoporous oxides, precipitation of alumina (i.e. aluminum oxide) from a liquid media may occur. This precipitation may be influenced by the amphoteric properties of alumina. Alumina is generally insoluble in neutral aqueous media, but can be dissolved in a strong aqueous acid, such as sulfuric acid (H2SO4) or nitric acid (HNO3), to form an acid aqueous solution of a salt, such as aluminum sulfate (Al2(SO4)3) or aluminum nitrate (Al(NO3)3). Alumina can also be dissolved in a strong base, such as sodium hydroxide (NaOH), to form a basic aqueous solution of a salt, such as sodium aluminate (Na2Al2O4 or NaAlO2). Conversely, the dissolution process can be reversed, and alumina can be precipitated from an aqueous solution by the addition of (1) an appropriate base to an acidic solution of an aluminum salt or (2) an appropriate acid to a basic solution of an aluminum salt. Alumina can also be precipitated from an organic solution of an aluminum alkoxide by adding water to this solution, thereby causing the aluminum alkoxide to hydrolyze into a hydrated form of alumina and an alcohol. Precipitated alumina, before dehydration, may exist in the form of a gel or sol.
Alumina can exist in a hydrated form and a dehydrated form. An example of a fully hydrated form of alumina may be described as alumina trihydrate, i.e. Al2O3.3H2O, or aluminum hydroxide, i.e Al(OH)3. Conventional forms of alumina may be prepared by precipitation from a liquid medium in the absence of a templating agent or structure directing agent to first form a hydrated form of alumina. This hydrated form of alumina may then be calcined under conditions to form various dehydrated forms of alumina. These dehydrated forms of alumina may be amorphous, semi-crystalline or crystalline. Activated alumina is a conventional and at least partially dehydrated form of alumina, which has been used in various catalytic operations as a stand-alone catalyst or catalyst support. Activated alumina includes the various transition aluminas, such as gamma-alumina, delta-alumina and theta-alumina.
Conventional forms of alumina, prepared by precipitation in the absence of a templating agent, followed by calcination at temperatures less than 1000° C., tend to have relatively high surface areas, for example, from 50 to 300 m2/g. However, calcination at temperatures higher than 1000° C. may result in a significant reduction in surface area. For example, calcination of a transition alumina at temperatures higher than 1000° C. may result in the transformation of the transition alumina into alpha-alumina, which has a surface area of less than about 20 m2/g.
The conventional forms of alumina which have relatively high surface areas, for example, from 50 to 300 m2/g, also tend to have broad pore size distributions, often multi-modal, particularly in the 3-15 nm range. This broad pore size distribution has been reported to be disadvantageous. For example, in the above-mentioned Pinnavia et al U.S. Pat. No. 6,027,706, at column 1, lines 48-58, it is stated that wide pore distribution limits the effectiveness of catalysts, absorbents and ion-exchange systems, and that very broad pore distribution is particularly limiting in the use of aluminas in petroleum refining. Also, in the above-mentioned Lee et al U.S. Patent Application Publication No. US 2005/0281734 Al, in paragraph [0005], it is suggested that mesoporous alumina with uniform porosity, high surface area, chemical stability and thermal stability is becoming more valuable than existing alumina with non-uniform pore size distribution.
However, mesoporous alumina made with templating or structure directing agents tends to lack thermal and hydrothermal stability. Consequently, applications of such mesoporous alumina have been limited to relatively low temperatures. In order for these types of alumina to be applicable under harsh and demanding hydrothermal conditions, such as those encountered in fluid catalytic cracking (FCC), hydrothermal stability of these aluminas needs to be improved.
The Pinnavaia et al patent (U.S. Pat. No. 6,027,706) and the Zhang et al article (Chem. Commun., 1998, 1185), describing the MSU-X series of materials, report that a significant portion of surface area and porosity of these materials are lost when these materials are calcined at temperatures as high as 500° C. In the Zhang et al article (Chem. Commun., 1998, 1185) stabilization of MSU-X materials is reported when such alumina is doped with Ce3+ or La3+. However, only improvement in thermal stability is reported; there is no mention of the hydrothermal stability of these modified materials.
The Shan et al article (Applied Catalysis A: General 254 (2003), pp. 339-343) reports that materials, characterized as Al-TUD-1, have improved thermal stability vs. those of the MSU-X type of porous aluminas. However, it appears that no report of hydrothermal stability for these types of Al-TUD-1 materials is available.
The Chester et al U.S. Pat. No 6,447,741 describes a mesoporous aluminophosphate material, which includes a solid aluminophosphate composition modified with at least one element selected from zirconium, cerium, lanthanum, manganese, cobalt, zinc, and vanadium. Example 2 of the Chester et al patent (U.S. Pat. No. 6,447,741) describes a cerium modified aluminophosphate material, and Example 4 describes a lanthanum modified aluminophosphate material.